An image forming apparatus such as a copying machine and a printer utilizing electrophotographic method is provided with an electrophotographic photosensitive member. In such image forming apparatus, the electrophotographic photosensitive member is rotated by a power transmitter, and synchronously with the rotation, operations such as electrification, exposure, development, transfer, and cleaning are repeated, thereby forming an image on a recording medium.
Specifically, in the image forming apparatus, the electrophotographic photosensitive member is electrically charged at its surface and then rotated while being irradiated by laser light for exposure, according to an image pattern, so that an electrostatic latent image is formed on the surface of the electrophotographic photosensitive member. Next, the latent image is developed by attaching toner to the photosensitive member. The toner attached to the electrophotographic photosensitive member is transferred to a recording medium. After the transfer of toner to the recording medium, the electrophotographic photosensitive member is rotated while a cleaning blade is pressed onto the surface of electrophotographic photosensitive member, so that remaining toner is removed.
The electrophotographic photosensitive member includes a metal cylindrical body on which a photosensitive layer is formed. The photosensitive layer includes a photoconductive layer formed on the cylindrical body using inorganic material, and a surface layer formed using inorganic material to coat the photoconductive layer. In such electrophotographic photosensitive member, respective thicknesses of the photoconductive layer and the surface layer are normally set to be substantially constant in the axial direction of the entire cylindrical body. Here, “substantially constant” means that a ratio (Te:Tc) of thickness (Tc) at the middle portion of the body to thickness (Te) at one end or the other end of the body is not more than 1.001:1.
In the electrophotographic photosensitive member, a heating means may be provided for heating the photosensitive layer from inside. This is for preventing image deletion by reducing moisture at the photosensitive layer.
However, when heating the electrophotographic photosensitive member by the heating means from inside, heat is unlikely to be released at the middle portion in comparison with heat at the end portions in the axial direction of the body, and thus temperature at the middle portion tends to be relatively higher than temperature at the end portions. Such tendency becomes significant when heating by the heating means is performed for a long time (e.g. when many sheets of paper are printed continuously).
Meanwhile, in the electrophotographic photosensitive member, charge transfer tends to be activated as the surface temperature becomes higher. Thus, in the above-described electrophotographic photosensitive member, charging characteristic (property to carry electric charge) tends to become lower at the middle portion than at the end portions in the axial direction. As a result, in the above-described electrophotographic photosensitive member, variation in charging characteristic in the axial direction and thus variation in image density are likely to be generated.